Electronically controlled microwave attenuator



Dec. 13,1960 R. H. HATCH 2,964,719

' ELECTRONICALLY CONTROLLED MICROWAVE AT'IENUATOR Filed Nov. 17, 1955CONTROL VOLTAGE FIG. 3

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l az/a o lNVENTOR ROBERT H. l-IATDH ATTORNEYS nited States PatentELECTRONICALLY CONTROLLED MICROWAVE ATTENUATOR Robert H. Hatch,Massapequa Park, N.Y., assignor, by mesne assignments, to the UnitedStates of America as represented by the Secretary of the Navy Filed Nov.17, 1953, Ser. No. 392,784 1 Claim. (Cl. 333-81) gated wave, both forthe purpose of amplitude modulation and for the purpose of maintaining aconstant energy output by regulation of a variable energy input. Whilein low frequency circuitry a lumped line impedance having variablecharacteristics will suflice for this purpose, control in a waveguidesystem is most effectively accomplished through a distributed impedance.In fact, it is desirable that the distributed impedance be co-extensivewith the cross sectional area of the Waveguide for greatesteiiectiveness in controlling the energy propagated therethrough. Toachieve a maximum range of control, however, the attenuating eifect ofsuch an impedance element must be readily removable.

As is well known, a waveguide system must be substantially free fromimpedance mis-matching to avoid creation of reflected waves andattendant lowering of efficiency. Accordingly, a further desideratum ofan impedance control device in a waveguide is that its'jdesign shouldnot require dimensional discontinuities within the waveguide. V

A continuous control of ultra high frequency signals propagated within awaveguide has heretofore been achieved with a confined ionizable gas inthe waveguide by the application of an ionizing voltage to a'pair ofelectrodes extending into the gas. Such electrodes have had a complexand expensive design tending to introduce substantial discontinuities inthe dielectric path ofthe microwave signal. Consequently, a difiicultyhas been experienced in achieving an impedance matching over anappreciabhrange of frequencies of the transmitted signal. Resort hasbeen taken to the insertion of a gaseous discharge tube within thewaveguide in an effort to confine therein a low pressure gas togetherwith electrodes for ionizing the gas. This construction has not resultedin even a substantial approach to the ultimate achievable range ofattenuation, presumably because the gaseous discharge tube cannot beconvenient-1y designed to have an interior dimension coextensive withthe interior dimensions of the waveguide section in which such tube isreposed. I p

Accordingly, it is the object of this invention to provide an'efiective,yet simple control of ultra high frequency energy propagated within aconfined dielectric medium.

It is a further object of this invention to provide an such a controlwherein a single electrode is employed 'ice 2. within a waveguidesection capable of confining the ionizable gas.

Still another object of this invent-ion is to provide a waveguidesection with resonant windows at either end to confine an ionizab-le gaswithin the section and to match the impedance of the section toimpedances of sections coupled thereto, while the gas is ionized tovaried degrees by means of an interior electrode.

As an exemplification of this invention, there is provided a waveguidesection sealed by a resonant window at either end to enclose an inert,ionizable gas which surrounds an emissive cold cathode spaced within thewalls of the waveguide section. In relation to a waveguide ofrectangular cross section with standard coupling flanges at either end,it is contemplated that the cathode should take the form of a planarstrip supported centrally in relation to the flanges and the walls ofthe waveguide and normal to the electric field of the ultra highfrequency signal. Control of energy propagated through this section isachieved by imposing a voltage upon the cathode negative in relation tothe walls of the waveguide section and variable over a range includingthe ionization potential of the gas. It will thus appear that thepresent invention may be embodied in a standard waveguide section simplymodified to incorporate the resonant sealing windows at either end andan emissive planar cathode intermediate such ends.

Further objects and advantages of the present invention will becomeapparent from a consideration of a preferred embodiment of the inventionwhich is herein described and which is represented upon the drawings,wherein:

Figure 1 is a front elevational view, displaying a cathode as seenthrough a resonant window;

Figure 2 is a cross sectional View taken along the line II--II in Figure1; and

Figure 3 is the bottom view of the device constructed in accordance withthe invention with a portion broken away to display the cathode.

Throughout the figures, like'numerals are used to desig nate like partsor portions. There is thus shown a space resonant cavity or chamber 10'which may comprise a section 11 of a standard waveguide of thehollow-pipe type. This waveguide section may have any suitable hollowconfiguration but, in the embodiment illustrated, has a conventionalrectangular cross section. It is requisite that the waveguide sectionhave electrically conductive walls defining an energy directing cavitywhich may be sealed ofi to confine an ionizable gas therein.

, To this end, the rectangular Waveguide section 11- is fitted at eitherend with coupling flanges 12 and 13. Each of these flanges has a collarportion 14 fitted snugly about the respective endwise portions 16 and 17of the waveguidesection and sealed thereto by soldering or other means.As seen in Figure 1, the collar portions 14 are provided with holes 15at their four corners to receive coupling bolts for securemeut to matingflanges of adjacent waveguide sections (not shown). Radially outward'ofeach collar portion. 14 an axially extending flange 18 defininganendwise opening cavity in which window assemblies 20' and 21 may bereceived.

Window assemblies 20 and 21 may each. comprise a frame 22 having araised bead 23 thereabout, a window 24 nested withinthe bead '23 andagainst the frame 22, and apertured mask 25. The Window 24 maybecomposed of any dielectric material relatively transparent to ultra highfrequency radiation, yet having sufiicient structural strength towithstand atmospheric pressure. As an examplqthe window '24 may becomposed of mica or glass in sheetform, so as to pass ultra highfrequency without substantial attenuation or reflection.

The mask 25', which fits within and is secured to the bead 23, providesa resonant aperture or iris 26, exposing the Window 24 for microwavetransmission. In the illustrated embodiment, the aperture 26 is shown tobe a centrally positioned slot with straight side edges 27 and circulartop and bottom edges-'28. An optimum design of the resonant aperture 26may be determined by principles well known in the art to provide amatching with the characteristic impedance of the waveguide sections orother elements to which either end of the waveguide section 11 may becoupled. The frame 22 supporting the resonant aperture may be lodgedsnugly within the axial flanges 18 and 19, so that an air tight sealwill be perfected at either end of the waveguide section 11 whenever itsflanges are coupled to mating flanges of other waveguide sections (notshown). Alternatively, the window assemblies 2t and 21 may be securedand sealed to the coupling flanges 12 and 13 by welding or the like.

When such a seal has been perfected, tubulation 30 may be firstconnected to an evacuation pump until a vacuum is produced within thecavity, and then it may be connected to a supply of inert ionizable gasor vapor at low pressure. The gas or vapor may suitable be composed ofneon, argon, zenon, mercury vapor, or a mixture. When the cavitycontains only a low pressure ionizable gas or vapor, the tubulation 30may be closed off at its end 31.

This invention further contemplates that within the cavity 10 will beplaced a cold cathode 34 of the electron emissive type. As the purposeof this emissive cathode 34 is to ionize the inert gas in a generallyuniform manner throughout the region of the cavity 10, the electrode isgiven a central position in relation to the surrounding conductive wallsof the cavity and to the endwise porcavity may be achieved. 1

tions 16 and 17 of the section. For this reason also, the

cathode preferably has a substantial axial dimension in relation to theaxial dimension of the cavity or section, where the axis is consideredto extend in the direction of wave propagation.

Because the cathode is comprised of a relatively conductive materialwhich would tend to disturb the transmission of an ultra high frequencysignal, it has been conceived that the cathode should take a position inthe dielectric medium where the electric field gradient of the signal issubstantially zero. Since, by conventional type of excitation, theelectric field E of an ultra high frequency signal passing through therectangular waveguide system 11 will extend from side to side within thecavity, or normal to the axial plane in which the section of Figure 2 istaken, the cathode 34 is placed in this same axial plane centrallydisposed between the side walls of the waveguide section 11. In thepreferred embodiment of the invention, therefore, there is employed aplanar or strip-like cathode 34 which is approximately equidistant fromall of the surrounding walls of the cavity in which it is placed and isdisposed within'those walls so as to be normal to the electric field Eof the ultra high frequency energy confined by the walls. At either endof the cathode 34, pins 39 and 40 are aflixed to provide a means ofsupport. These pins 39 and 40 composed of electrically conductivematerial are led out through sealed ports 41 and 42, respectively, inthebottom wall of the waveguide section, and are insulated from the bottomwall and the ports by means of bushings 43 and disks 44 composed ofinsulating material. A control voltage from whatever source desired mybe applied across leads 45 and 46 connecting, respectively, to thecathode 34 through the pin 41 and to the metallic walls of cavity 10.

A zone of ionization will be created by application of a control voltageat 45 46, which will substantially pervade and extend throughout theentire region of the cavity 19. The ionization thus created will besubstantially uniform by reason of the central positioning of theemissive cathode 34. As aconsequence, a high degree of control In use,the waveguide section 11, is coupled between other waveguide elementswhich generally will be waveguide sections of similar configuration. Thecoupling of the sealed waveguide section 11 may be accomplished bybolting together the coupling flanges 12 and 13 with mating'flanges ofadjacent'waveguide sections.- Ultra high frequency is then introducedwithin the waveguide system and enters and departs from the sealedwaveguide section through the resonant windows 24. Assuming first thatno control voltage is imposed upon the leads 45-46, the ultra highfrequency energy will pass through the section 11 without an appreciableattenuation, just as if the section were of standard design and devoidof control features. This results from the employment of resonantwindows 24, the use of a standard waveguide 11 to form a cavity, and thepositioning of the cathode 34 normal to the electric field. When anegative voltage is applied to the cathode 34 sufficient to causeemission of electrons, the inert gas filling the cavity is ionized bythe collison energy of the electrons moving toward the more positivelycharged walls of the cavity. In accordance with the wellknown theory ofgaseous discharge devices, the degree of ionization within the cavitymay be rapidly varied in proportion to a variable voltage applied to thecathode. The degree of attenuation of the propagated ultra highfrequency energy is a direct function of the dielectric constant of thepropagated medium and is, therefore, a function of the differencebetween the dielectric constant of the medium when not ionized and aquantity which is directly proportional to the number of chargedparticles within a unit volume and the square of the unit charge of eachparticle and which is inversely proportional to the mass of each chargeand a function of the signal frequency. Thus, the attenuation of theultra high frequency signal may be accomplished by a varying degree ofionization.

Utility of this device in modulating a guided ultra high frequency waveby an alteration of the dielectric constant will be apparent to thoseskilled in this art. Briefly, however, this device may be used foramplitude, phase, or frequency modulation of the propagated wave, or forregulating the energy thereof. Because the waveguide section employed inthis embodiment of this invention is basically of a standard design, itmay be manufactured in accordance with prevailing practice, with thefurther steps of assembling the simple cathode structure interiorly ofthe section, afiixing the sealing window assembly at either end of thesection, and filling the section with a suitable gas. Upon theseconsiderations it will be apparent that there is provided a relativelyinexpensive, utilitarian device serving a variety of purposes whenemployed in conjunction with a waveguide system.

The particular embodiment of this invention, which has here been shownand described in detail, is intended merely to exemplify the principlesand practice of this invention. Accordingly, as it will be obvious tothose skilled in the art that changes and modifications may be madetherein without a substantial departure, it is intended in the appendedclaim to cover all such changes and modifications as fall within thetrue spirit and scope of this invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

In,a waveguide system within which electromagnetic waves may bepropagated, a sealed waveguide section enclosing a quantity of ionizablegas, said waveguide section having conductive interior walls defining arectangular cavity, a strip-like electron emissive electrode supportedwithin said section in an equidistantly spaced and electricallyinsulated relation to the walls thereof, said electrode lying in a planeparallel to opposite interior walls of said section and extendinglengthwise in relation thereto, and means for applying a potentialbetween said Walls and said electrode, thereby to ionize said gasuniformly by reason of said equidistantly spaced relationship.

References Cited in the file of this patent UNITED STATES PATENTS 5Fiske Sept. 3, 1946 Korman June 10, 1947 Gorn Apr. 25, 1950 Depp May 16,1950 10 Evans Nov. 28, 1950 Malter Nov. 27, 1951 6 Fiske Dec. 4, 1951Wilson Apr. 28, 1953 Townes Apr. 26, 1955 I Townes Apr. 26, 1955 ZaleskiOct. 2, 1956 Hatch Nov. 13, 1956 Goldstein Sept. 8, 1959 FOREIGN PATENTSGreat Britain Jan. 27, 1947

